Check run_command() return code properly

run_command() returns 0 for success, 1 for failure. Fix places which
assume that failure is indicated by a negative return code.

Signed-off-by: Thomas Betker <thomas.betker@rohde-schwarz.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>